Digital radio communication now occupies the important position in the field of communication by virtue of the technological development thereof. In an attempt to pursue higher speed of communication, study has being put forward toward communication using impulse modulation scheme. Impulse modulation scheme involves the problem of being ready to undergo interference from other systems and hence become instable during communication because of its occupation over a broadband frequency. Meanwhile, because of band occupation, there is a difficulty in multiplexing a plurality of channels.
However, as means for solving this, there are those as described in U.S. Pat. No. 5,677,927, for example. FIG. 44 shows the conventional communication apparatus described in U.S. Pat. No. 5,677,927.
In FIG. 44, a subcarrier generator & modulator 4401 generates a modulation subcarrier signal to be modulated with an information signal, and outputs the modulation subcarrier signal to a subcarrier time modulator 4402. By the subcarrier time modulator 4402, an encoded timing signal is modulated to generate a modulated encoded timing signal. The timing signal is radiated in the form of an electromagnetic pulse, at a transmission antenna 4404 through an output stage 4403.
With the above configuration, the communication apparatus in the prior art makes an impulse signal into channels by the simultaneous uses of subcarriers different in frequency or waveform. By using separate subcarriers at individual channels, communication is made feasible simultaneously at a multiplicity of independent channels.
However, in the configuration of U.S. Pat. No. 5,677,927, because communication is by a broad use of band on the impulse modulation scheme, there is a difficulty in avoiding against the system that issues high output signals in a part of the band, resulting in readiness to undergo influence.
Meanwhile, because the interval of subcarrier frequencies is great in distance (interval equal to or greater than 500 MHz), there is a conspicuous difference appearing in frequency-based radio wave propagation characteristic. Namely, concerning the higher-frequency subcarrier, there is limitation in the area for transmission as compared to the lower-frequency subcarrier. Thus, there is greater effect of shadowing. Besides, attenuation is high at around shields such as walls. This is the case from the fact that generally, at lower frequency band, communication is favorable with reduced circuit disconnections and broader communication area while, at higher frequency band, circuit disconnection is higher in rate and communication area is narrower.
These can be responsible for the followings.
Free space propagation loss: loss increases with increasing frequency. Narrowing communication area.
Transmission characteristic: loss of transmission through a shield increases with increase of frequency.
Diffraction effect: diffraction effect decreases and shadowing influence increase with increase of frequency.
Consequently, the communication system using impulse modulation scheme has the problem of the above conspicuous setbacks, as compared to the communication system in the prior art having a carrier interval of approximately several MHz to several tens MHz.